Conventional helical screw piles include a plurality of helical plates arranged on a longitudinal shaft having a square cross section. Typically, the helical plate with the largest diameter is disposed towards the top of the shaft and the helical plate with the smallest diameter is disposed towards the bottom of the shaft that first penetrates the ground. Turning to FIG. 1, a conventional screw pile 100 includes a plurality of helical plates 120, 122, 124 arranged in descending order from the top 114 of the shaft 112 to the bottom 116 such that the helical plate 120 with the largest diameter closest to the top end 114 of the hydraulic motor 118 and the helical plate 124 with the smallest diameter adjacent the tip 130 of the pile 100.
Inter-helix spacing is critical to the design of the helical screw pile. Inter-helix spacing is the distance between each of the helical plates. Standard practice is to space the helical plates as a function of plate diameter so that the spacing between the uppermost plate and the middle plate is greater than the spacing between the middle plate and the lowermost plate. The most common inter-helix spacing in the industry provides spacing between the first lowermost plate and a second plate being less than the spacing between the second plate and the third uppermost plate.
A conventional screw pile shown in FIG. 1, where the helical plate 124 at the bottom 116 of the shaft 112 has the smallest diameter, the distance L1 between the lowermost helical plate 124 and the helical plate 122 directly above is less than the distance L2 between the helical plate 120 and its adjacent helical plate 122 is greater than L1.
With this configuration, the smallest helical plate 124 adjacent the tip 130 of the pile 100 is the first helical plate that disturbs, or breaks, the surface when the pile 100 is inserted into the ground. As the helical plate diameter increases, the amount of torque required to insert the pile 100 increases. Thus, when the top helical plate 120 with the largest diameter is driven into the ground, the greatest amount of torque that is required for rotating the helical plate 120 is compromised because of the force or impact on the smaller helical plates 120, 122, 124 already positioned below the ground surface.
In response to this recognition, certain devices have been designed to better withstand the rigors of digging large holes in the ground. Examples of prior art are disclosed in U.S. Pat. No. 2,603,319 to Dyche, U.S. Pat. No. 7,635,240 to Gantt, Jr., and U.S. Pat. No. 7,494,299 to Whitsett which are hereby incorporated by reference.